nf-graph: better behavior when setting yMin, yMax to null

Previously, setting `yMin` or `yMax` to `undefined` caused

> Error: Invalid value for <g> attribute transform="translate(520, NaN)"

There was no way to set `yMin` or `yMax` to "something if it's defined,
or default behavior otherwise". This now treats the following as
equivalent:

```hbs
{{!-- no yMax specified: --}}
{{nf-graph}}

{{!-- yMax set to undefined: --}}
{{nf-graph yMax=somePropertyThatIsUndefined}}
```

app/components/nf-graph.js

@@ -120,6 +120,7 @@ var minProperty = function(axis, defaultTickCount){
   var _Axis_tickCount_ = axis + 'Axis.tickCount';
   var _ScaleFactory_ = axis + 'ScaleFactory';
   var __Min_ = '_' + axis + 'Min';
+  var __didOverride_ = '_didOverride_' + axis + 'Min';
   var _prop_ = axis + 'Min';
 
   return Ember.computed(
@@ -131,33 +132,38 @@ var minProperty = function(axis, defaultTickCount){
       var mode = this.get(_MinMode_);
       var ext;
 
-      if(arguments.length > 1) {
-        this[__Min_] = value;
-      } else {
-        var change = function(val) {
-          this.set(_prop_, val);
-        }.bind(this);
+      if(value != null) {
+        this[__didOverride_] = true;
+        return this[__Min_] = value;
+      }
 
-        if(mode === 'auto') {
-          change(this.get(_DataExtent_)[0] || 0);
-        }
+      if(this[__didOverride_]) {
+        return this[__Min_];
+      }
+
+      var change = function(val) {
+        this[__Min_] = val;
+      }.bind(this);
 
-        else if(mode === 'push') {
-          ext = this.get(_DataExtent_)[0];
-          if(!isNaN(ext) && ext < this[__Min_]) {
-            change(ext);
-          }
+      if(mode === 'auto') {
+        change(this.get(_DataExtent_)[0] || 0);
+      }
+
+      else if(mode === 'push') {
+        ext = this.get(_DataExtent_)[0];
+        if(!isNaN(ext) && ext < this[__Min_]) {
+          change(ext);
         }
+      }
 
-        else if(mode === 'push-tick') {
-          var extent = this.get(_DataExtent_);
-          ext = extent[0];
+      else if(mode === 'push-tick') {
+        var extent = this.get(_DataExtent_);
+        ext = extent[0];
 
-          if(!isNaN(ext) && ext < this[__Min_]) {
-            var tickCount = this.get(_Axis_tickCount_) || defaultTickCount;
-            var newDomain = this.get(_ScaleFactory_)().domain(extent).nice(tickCount).domain();
-            change(newDomain[0]);
-          }
+        if(!isNaN(ext) && ext < this[__Min_]) {
+          var tickCount = this.get(_Axis_tickCount_) || defaultTickCount;
+          var newDomain = this.get(_ScaleFactory_)().domain(extent).nice(tickCount).domain();
+          change(newDomain[0]);
         }
       }
 
@@ -172,6 +178,7 @@ var maxProperty = function(axis, defaultTickCount) {
   var _ScaleFactory_ = axis + 'ScaleFactory';
   var _MaxMode_ = axis + 'MaxMode';
   var __Max_ = '_' + axis + 'Max';
+  var __didOverride_ = '_didOverride_' + axis + 'Max';
   var _prop_ = axis + 'Max';
 
   return Ember.computed(
@@ -183,33 +190,38 @@ var maxProperty = function(axis, defaultTickCount) {
       var mode = this.get(_MaxMode_);
       var ext;
 
-      if(arguments.length > 1) {
-        this[__Max_] = value;
-      } else {
-        var change = function(val) {
-          this.set(_prop_, val);
-        }.bind(this);
+      if(value != null) {
+        this[__didOverride_] = true;
+        return this[__Max_] = value;
+      }
 
-        if(mode === 'auto') {
-          change(this.get(_DataExtent_)[1] || 1);
-        }
+      if(this[__didOverride_]) {
+        return this[__Max_];
+      }
+
+      var change = function(val) {
+        this[__Max_] = val;
+      }.bind(this);
 
-        else if(mode === 'push') {
-          ext = this.get(_DataExtent_)[1];
-          if(!isNaN(ext) && this[__Max_] < ext) {
-            change(ext);
-          }
+      if(mode === 'auto') {
+        change(this.get(_DataExtent_)[1] || 1);
+      }
+
+      else if(mode === 'push') {
+        ext = this.get(_DataExtent_)[1];
+        if(!isNaN(ext) && this[__Max_] < ext) {
+          change(ext);
         }
+      }
 
-        else if(mode === 'push-tick') {
-          var extent = this.get(_DataExtent_);
-          ext = extent[1];
+      else if(mode === 'push-tick') {
+        var extent = this.get(_DataExtent_);
+        ext = extent[1];
 
-          if(!isNaN(ext) && this[__Max_] < ext) {
-            var tickCount = this.get(_Axis_tickCount_) || defaultTickCount;
-            var newDomain = this.get(_ScaleFactory_)().domain(extent).nice(tickCount).domain();
-            change(newDomain[1]);
-          }
+        if(!isNaN(ext) && this[__Max_] < ext) {
+          var tickCount = this.get(_Axis_tickCount_) || defaultTickCount;
+          var newDomain = this.get(_ScaleFactory_)().domain(extent).nice(tickCount).domain();
+          change(newDomain[1]);
         }
       }